This disclosure relates to semiconductor fabrication, and particularly to wafer overlay.
Semiconductor fabrication involves multiple varied steps and processes. One typical process is known as photolithography (also called microlithography). Photolithography uses radiation, such as ultraviolet or visible light, to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using semiconductor fabrication techniques including photolithography, etching, film deposition, surface cleaning, metallization, and so forth.
Exposure systems (also called tools) are used to implement photolithographic techniques. An exposure system typically includes an illumination system, a reticle (also called a mask) or spatial light modulator (SLM) for creating a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle or SLM with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle pattern onto the wafer. For accurate projection, it is important to expose a pattern of light on a wafer that is relatively flat or planar, preferably having less than 10 nanometers of height deviation.